Compound liquid flow meters have parallel flow paths, one of which includes a turbine meter accurate over a wide range of relatively high flow rates, but inaccurate below a known minimum rate. The other flow path includes a meter accurate at low flow rates, but having a narrow range with a low maximum rate of flow greater than the minimum flow rate of the turbine meter. It is essential that the flow path through the turbine meter be closed by a valve during the low range of flow rates where that meter is inaccurate. It has previously been considered necessary to have the valve quick-acting in response to the rate of flow, so that it opens quickly when the flow rate exceeds the minimum accurate flow rate of the turbine meter, and so that it closes quickly when the flow rate falls below that minimum.
Many quick-acting valve mechanisms have been proposed in the prior art. See for example Masson et al. U.S. Pat. Nos. 3,677,084 and Pelt, 4,100,800. The prior art quick-acting valve mechanisms are complex. Masson et al. uses a combined cam and toggle mechanism. Pelt uses a complex mechanism involving two cams and an inclined plane. The prior art compound meters including quick-acting valve mechanisms have reduced accuracy during the crossover range of flows during which the valve is opening or closing. This lack of accuracy is recognized in the standard for such meters published by The American Water Works Association and identified as their Standard C-702-78. That standard allows compound meters to have a minimum accuracy as low as 90% in the crossover range where the valve is opening or closing, but requires that the total crossover range of flow rates where accuracy is below 97% be limited (e.g., 20 gals. per min. for a 3 in. meter).